1. Field of the Invention
This invention relates to a suspension system for vehicles and more particularly to a suspension system in which two damping devices are arranged in series between wheel side members and body side members.
2. Description of the Prior Art
In general, in the vehicle suspension system a damping device comprising a spring member (coil spring, etc.) and a damper member (shock absorber, etc.) is arranged between the wheel side members and the body side members for controlling the oscillation of wheels. As to the damping force of a damper member of this damping device, for the purpose of improving running stability and comfortableness to ride in it is considered desirable to make damping force larger at the area of low oscillation frequency, namely, the area of oscillation frequency caused by the operation of a driver (2-3 Hz or less) and to make damping force smaller at the area of high oscillation frequency, namely, the area of oscillation frequency caused by the road surface stimulus, etc. (about 5 Hz or higher). For this purpose, it has been known to use a shock absorber of damping force variable type for the damper member but this shock absorber has disadvantages in that it is complicated in the composition of its control part, etc., high in manufacturing cost and lacks responsiveness.
Also, it has been known to arrange two damping devices in series, each comprising a shock absorber and a coil spring, between the wheel side members and the body side members, as disclosed by the Japanese Utility Model Registration Application Laying Open Publication No. 62-103713 (damping device of this kind is hereinafter referred to as multi-damper). In this case of this multi-damper, if it is so set that damping coefficients of shock absorbers in the two damping devices are different, it is possible to change damping force into two values according to the oscillation frequency. In the Publication No. 62-103713, a shock absorber of damping force variable type is used for one of two damping devices.
Damping force by the shock absorber is not determined only by the damping coefficient but relates to the length, speed, etc. of expansion and contraction by the shock absorber. Therefore, in the multi-damper mentioned above, the easygoing combination of the shock absorber and the coil spring (two shock absorbers of different damping coefficient and two coil springs of the same spring constant, for example) does not present a big difference in damping force between the area of low oscillation frequency and the area of high oscillation frequency and cannot improve the running stability and comfortableness to ride in on a high level.